Psychophysics of the basic sound dimensions

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Psychophysics of the basic sound dimensions

• Perfecto Herrera
• Music Perception and Cognition

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Physical and perceptual features of sounds

• Waveform amplitude -gt Loudness
• (the larger, the louder)
• Waveform period -gt Pitch
• (the longer, the lower)
• Waveform shape -gt Timbre
• (the more rippled far from sinusoidal-, the
  richer)

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Psychophysical Laws

• Mathematical expressions relating a physical
  property with a perceptual sensation
• Response f (sensory stimulation)
• Is f linear, potential, exponential?
• Are sensations totally independent?
• Collect subjective judgments when presenting
  different intensities, pitches, along a single
  dimension
• Find the best fit between the physical magnitudes
  and the perceptual estimations
• Be careful with physiological constraints -gt
  Frequency resolution of the ear, Energy
  integration, Firing rate limitations

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Psychophysical Laws

• Which is the absolute threshold for a sensation
  to happen?
• Is it interacting with another physical feature?
• Which is the relative threshold (just noticeable
  difference, JND)?
• Is it fixed for all the range of physical
  stimulation values?

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Differential thresholds

• Which is the minimum difference that can be
  perceived with relation to a given sensation?
• Just noticeable difference (JND) or
  Differential threshold
• Listen one sound, then another, then decide if
  they are the same or they are different.
• The first difference or jump that is noticed by
  more than 75 of the listeners is considered to
  be the JND for that sensation

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Absolute and differential thresholds
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Psychophysical Laws

• Fechners Law R k log(I)
• R is the sensation, I is the physical property, k
  a constant to be found or adjusted from the data
• Webers Law ?I/I k
• The just-detectable change in stimulus intensity
  (jnd or DL) is proportional to the intensity
• Stevens Law R kIp

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• Perception of Loudness

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Intensity (physical magnitude)

• I p2/?c
• p is the pressure of the air in a given point of
  space and time
• ? is the density of the air
• c is the speed of sound in the medium where it is
  being transmitted
• ?c 40 dines per centimeter.

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Compression and rarefaction
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Loudness

• The subjective sensation generated by the
  intensity of the air pressure is called Loudness

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Hearing Loudness thresholds

• MAF (minimum audible field) pressure measured
  in the free field where a listeners head would
  be. The sound source is directly in front of the
  listener.
• MAP (minimum audible pressure) pressure
  measured in the ear canal. Thresholds are
  measured in one ear only.
• Differences in the two measures are due to some
  binaural advantage, outer-ear filtering (mid
  frequencies), and physiological noise (low
  frequencies).

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Absolute thresholds

• Minimum audible pressure
• 0.0002 dines/cm2
• 0.0002 microbars
• 20 micropascals 10-16 W/cm2
• Pressure causing pain, not sound sensation 20hPa
• 1hPa 1 x 108 micropascals
• The atmospheric presssure is measured in
  hectopascals
• Normal athmospheric pressure 1013 hPa
• 130dB 63Pa 0.63 hPa
• A sudden drop of 1hPa storm approaching- may
  cause our ears hurt (gt130dB change) !!!

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Hearing Level

• Threshold of hearing, relative to the average of
  the normal population.
• For example, the average threshold at 1 kHz is
  about 4 dB SPL. (dBHL)
• HL expresses the amount the threshold has been
  raised compared to the normal population
• It deteriorates with age, drug and food
  consumption and behaviour patterns

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The dBSPL

• Unit preferred to measure the Sound Pressure
  Level
• It usually ranges from 0 to 130
• Uses the minimum audible pressure as reference
  value (P0 )
• What does 0dBSPL mean? No pressure?
• dBSPL 20 log(P/P0).
• dBs are not additive (20dB20dBltgt40dB)

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Loudness Scaling

• Can we order loudness sensations (i.e., this
  sound has twice the loudness than another one)?
• L k I0.3 (I Intensity Stevens Law)
• So, a 10-dB increase in level gives a doubling in
  loudness.
• This provides the basis for the loudness scale,
  measured in Sones.
• A 1-kHz at 40 dB SPL is defined as having a
  loudness of 1 Sone. So, a 1-kHz tone at 50 dB SPL
  has a loudness of about 2 Sones (twice as loud),
  _at_60dB -gt 4 Sones, _at_70db -gt 8 Sones

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Equal Loudness Contours

• 1 kHz is used as a reference. By definition, a
  1-kHz tone at a level of 40 dB SPL has a loudness
  level of 40 phons.
• Any sound producing the same loudness (no matter
  what its SPL) as the reference tone also has a
  loudness level of 40 phons.
• Sones versus Phons (?)
• Equal-loudness contours are produced using
  loudness matching experiments

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Equal Loudness estimation
Skovenborg, Quesnel, Nielsen (2004). Loudness
assessment of music and speech, 116th convention
of the AES
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Equal Loudness Contours
80-100 phon curves are flatter -gt consequences
for mixing?
Low and high frequencies have to be raised in
intensity, specially when listening at soft
levels -gt consequences for home amplifiers?
a.k.a Isophonic curves or Fletcher Munson
curves
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Equal Loudness Contours

• Interpret / Explain (voluntary homework)
• A tone has 64 sones
• A tone has 60 phones
• A tone has 60 dBSPL
• Two tones are isophonic
• If 1kHz _at_ 50 phones gives 2 sones, may 100Hz _at_ 40
  phones give 2 sones?
• Which one has a higher intensity, a tone of 40
  phones or a tone of 50 phones?

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Loudness weighting scales

• Filters are used in loudness meters to compensate
  for the changes in loudness as a function of
  frequency
• dB(A) A weighting 40 phon curve (approx.)
• dB(B) B weighting 70 phon curve (approx.)
• dB(C) C weighting essentially flat -high
  sound pressure levels with LF presence.

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Loudness and duration

• Energy integration time lt 200ms
• The longer the tone, the louder, up to 150ms

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Differential thresholds

• Just noticeable differences for a 1kHz tone and
  for white noise they have been estimated using
  the modulation method (modulation rate 4Hz). For
  the sinusoidal case, the sensitivity increases
  with the intensity level of the tone

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Neural coding of intensity

• Schematic illustration of input-output functions
  on the basilar membrane (response measured as
  movement of the BM)
• The solid line shows a typical function in a
  normal ear for a sinewave input with frequency
  close to the characteristic frequency
• The dashed line shows the function that would be
  observed in an ear in which the active mechanism
  was not operating

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Neural coding of intensity

• Firing rates of single auditory neurons as a
  function of stimulus level (rate-versus-level
  functions)
• In each case, the stimulus was a sinewave at the
  characteristic frequency of the neuron. Curves
  (a), (b), and (c) are typical of what is observed
  for neurons with high, medium, and low
  spontaneous firing rates, respectively

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Neural coding of intensity

• It is mainly coded by means of firing rates the
  higher the intensity the higher the rate but
• A single neuron dynamic range (_at_ 35 dB) does not
  explain dynamic range of auditory system (_at_ 140
  dB) so..
• The outputs of many different types of cells
  together may determine perception of loudness
• Loudness increases as several well-separated
  neurons fire around the same moment (remember the
  critical band issue)

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Loudness of complex sounds

• Does loudness increase adding energy at any place
  in the spectrum?

• We have to consider the frequency resolution of
  our hearing system

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Loudness of complex sounds

• Peripheral processing (filtering according to the
  outer and middle ear specificities)
• Computation of the excitation pattern considering
  the masking effects (cochlea neural firing
  approximation)
• 3. Conversion of the excitation pattern into
  band-specific loudness computation.
• 4. Summation of the specific band-loudness into
  the final loudness value
• Loudness increases (additively) only when there
  is energy beyond the critical band